1. Field of the Invention
The present invention relates to a process for preparing superconducting thin films and an apparatus for carrying our the process. More particularly, the present invention relates to a process for preparing a thin film of high temperature compound oxide superconductor on a very large area of a substrate by a magnetron sputtering method and an apparatus for carrying out the process.
2. Description of the Related Art
Bednorz and Muller discovered (La,Sr).sub.2 CuO.sub.4, which exhibits superconductivity at 30K, in 1986 (Z. Phys. B64, 1986, page 189) C. W. Chu et al. discovered, in the United States of America, another superconducting material, YBa.sub.2 Cu.sub.3 O.sub.7-.delta., having a critical temperature of about 90K, in 1987 (Physical Review Letters, Vol. 58,No. 9, page 908). Maeda et al. discovered so-called bismuth type superconducting material (Japanese Journal of Applied Physics, Vol. 27, No. 2, 1987, pages 1209-1210). In this specification, the term "high-temperature compound oxide superconductor" means compound oxides having a critical temperature above 30K.
The temperature compound oxide superconductors were obtained in the form of a sintered mass prepared by powder metallurgy. However, the sintered mass showed relatively poor superconducting properties, particularly low critical density (Jc). In order to increase the critical current density (Jc), a variety of thin film forming processes of these materials have been studied. Superconducting thin films of these compound oxides can be prepared by a variety of vapor deposition techniques such as vacuum-evaporation, sputtering, and molecular beam epitaxy (MBE) on single crystal substrates such as SrTiO.sub.3, MgO or the like. Remarkable improvements in the critical current density of these compound oxides have been achieved in their thin films.
Recently, many studies concerning actual application of the thin films to electronics devices and circuits have been reported and there is strong, steady demand to provide thin films of high-temperature oxide superconductors whose quality is guaranteed and which can be used in the research and development of such devices. In addition, there is a strong demand to provide a thin film of high-temperature oxide superconductor having desired dimensions, particularly having a large area.
Applicants have proposed a sputtering method for preparing thin film of superconducting oxide in which a substrate and a target do not face each other so that the thin film is not damaged by secondary electrons or charged particles of high energy emitted from the target while the thin film is deposited on the substrate.
However, it is difficult to prepare a thin film of high-temperature oxide superconductor on a large area substrate by the conventional film forming techniques such as sputtering or by the applicants' method mentioned above, so that the obtained thin films do not show uniform or homogeneous quality all over the large area. For this reason, it is not easy to fabricate electronics devices and/or circuits on a single thin film.
Therefore, an object of the present invention is to solve the above problems and to provide an effective process for preparing a large thin film of high-temperature compound oxide superconductor of uniform and homogeneous quality on a large area substrate by a magnetron sputtering method and an apparatus for carrying out the process.